Ketchup On Burgers...

...is a cardinal sin here in Texas. Anyone know why this is? It's mustard and/or mayo here -- along with lettuce, tomatoes, pickles, onions, cheese and bacon, of course -- but never ketchup. It never occurred to me until a conversation I had last week with a coworker from Colorado, but I realized that you will never catch a Texan intentionally putting ketchup on their burger.